Back to the Root—The Role of Botany and Plant Physiology in Cannabis Testing, Part I: Understanding Mechanisms of Heavy Metal Uptake in Plants: Page 4 of 4

March 6, 2020
Volume: 
3
Issue: 
2
Abstract / Synopsis: 

This article series will explore the effects of plant physiology on testing, including an examination of matrix effects, how specific types of analytes are transported through plant tissues, and synthesis pathways for compounds of interest such as terpenes and cannabinoids. This installment focuses on the physiology of heavy metal transport and translocation into and within plant tissues. It includes a brief explanation of the plant vascular system, how it functions, and how ions enter and move within this system. It finishes with a discussion of how nonessential, toxic substances (that is, heavy metals) enter plant systems, where they accumulate, and potential implications for testing.

Cannabis and Heavy Metal Research

While there is much research into heavy metal uptake, accumulation, and translocation in edible crop species for obvious consumer safety reasons, much less is known about heavy metal uptake in members of the genus Cannabis. What research has been done has focused primarily on low cannabidiol (CBD) varietals, commonly referred to as “hemp,” with little to no research available on the high tetrahydrocannabinol (THC) varietals mainly found in recreational and medical markets. While there is still much debate on how to botanically classify the various species in the genus Cannabis and indeed exactly how many species exist, research on both high CBD and THC chemotypes is critical.

Based on what research there is, hemp (Cannabis sativa) is thought to be an excellent candidate for phytoremediation of heavy metal contaminated soils (4). Some work was done on the phytoremediation potential of hemp around the Chernobyl site in 1998 that had promising results, and an abstract from the International Botanical Congress of 1999 indicated that C. sativa can accumulate lead and uranium in aerial parts (4). More recently, researchers have explored how effectively hemp is at accumulating chromium, cadmium, and nickel. The researchers also investigated metal translocation to the stems and leaves by comparing the concentration of metals in the roots, stems, and leaves of plants grown in soils with different heavy metal concentrations.

Interestingly, Citterio and colleagues found that hemp predominantly accumulates these heavy metals in its roots (4). This has serious implications if whole-plant testing becomes standard practice in heavy metal analysis for hemp. The researchers also investigated if heavy metals had any impact on hemp plant growth. They found that at experimentally elevated levels (Cd 80 µg/g, Ni 115 µg/g, and Cr 140 µg/g), heavy metals in the soil do not significantly interfere with hemp growth (4). However, there were measurable stress responses, such as elevated levels of phytochelatins and glutathione as well as the production of a novel phytochelatin in treated plants (4).

Further evidence that hemp plants sequester heavy metals in their roots was found in a study by Linger and colleagues that looked at the uptake and impact of cadmium on growth and photosynthesis in hemp (C. sativa). Researchers found that up to 17 mg/kg of available cadmium can be tolerated by hemp plants without any major effects to biomass production (5). They found at values near 72 mg/kg that cadmium began to have a significant impact on plant growth and photosynthesis. Both studies used a fiber variety of hemp, but Linger and colleagues had a lower soil pH while Citterio and colleagues had a soil pH that was slightly basic, which could have contributed to greater cadmium uptake into plant tissues at comparable cadmium soil concentrations.

Conclusion

Our understanding of how the genus Cannabis uptakes, translocates, or sequesters heavy metals is still on the scientific frontier. Research on other heavy metals of interest such as mercury, lead, and arsenic is still needed. It would also be useful to determine the mechanisms through which Cannabis species uptake, sequester, and control the translocation of different heavy metals since much of this information is speculated based on other plant species, namely agricultural crops or model organisms. Finally, it is important that chemotype (high CBD, high THC, fiber producing, and so on) be treated as an important variable, with studies done on different varietals. We still do not have a clear picture as to how to classify the different members of the genus Cannabis, so we cannot assume that different chemotypes will respond to the same external factors in the same way.

In the next installment we will explore pesticides, and how they interact with pre-existing plant defense systems.

References: 
  1. R.F. Evert and S.E. Eichhorn, Raven Biology of Plants, 8th Edition (W. H. Freeman, Macmillan, 2013) pp. 708–721.
  2. J.R. Peralta-Videa, M.L. Lopez, M. Narayan, G. Saupe, and J. Gardea-Torresdey, Int. J. Biochem. Cell Biol. 41(8–9), 1665–1677 (2009). 
  3. R.A. Wuana and F.E. Okieimen, ISRN Ecol, A 402647, 1–20 (2011).
  4. S. Citterio, A. Santagostino, P. Fumagalli, N. Prato, P. Ranalli, and S. Sgorbati, Plant Soil, A 256, 243–252 (2003). 
  5. P. Linger, A. Ostwald, and J. Haensler, Biol. Plant. A 4, 567–576 (2005).

About the Author

Gwen Bode, B.S., is an aspiring doctoral candidate and botanist with a strong chemistry background. As an undergraduate at Eastern Washington University she investigated the vitamin content of a wild edible plant via HPLC. She has since worked at the front line of the cannabis testing industry, integrating her botanical knowledge with the practical aspects of analytical testing. Direct correspondance to: [email protected]

How to Cite this Article

G. Bode, Cannabis Science and Technology 3(2), 26–29, 45 (2020).